In the machining operation of the bars in automatic lathes, said bars tend to buckle in the guide-bar tubes and/or in the headstock of the lathe. This buckling occurs more particularly at high rotational speeds of said bars due to the fact that the latter have a great length and a relatively small diameter. The risk of buckling is enhanced when the bar is being pushed towards the headstock of the lathe. Due to this buckling (FIG. 1), bar 3 will float in guide-bar tube 1 and in the headstock of the lathe. Said floating of the bar creates balance errors and vibrations which are transmitted up to the headstock of the lathe, thus producing inexactitudes in the machining of the bar, such as concentricity errors, ovalities and surface quality defects of the machined pieces. FIG. 1 shows a device where bar 3 is free between the head 11a of feed piston 2 and chuck 5 of the lathe. This is the most unfavorable case since the length of deformation is maximum. FIG. 2 shows the effect of a centering ring which is disposed between the exit of guiding tube 1 and the entry of head stock 4. Such a centering ring is the object of U.S. Pat. No. 4,507,992 of the same inventor. Said centering ring 7 provides an improvement in that bar 3, which is held by the chuck of the lathe while being centered and supported at the rear by the feed piston, is kept from floating at the location of ring 7. In this case, however, the bar can still be floating between feed piston 2 and ring 7 and between the latter and the chuck of the lathe, respectively.
The ideal solution would consist in guiding the bar over its entire length in the headstock of the lathe by a headstock reducing sleeve corresponding to the material diameter and by a hydrodynamic guide-bar tube whose internal diameter is dimensioned optimally with respect to the diameter of the bar to be guided. Such a solution, however, is difficult to apply in practice, since it would require a guiding tube for each diameter and each profile of bars to be machined, which entails costs and start-up times which are inacceptable in practice.
FIG. 3 suggests still another improvement which consists in placing a headstock reducing sleeve 8 between centering ring 7 and headstock 4 of the lathe. This procedure is being used by the inventor in the device which is the object of European Patent Application No. 88810416.3. This principle has the drawback that the diameter of the feed piston must be equal to or smaller than that of the bar to be machined in order to allow the penetration of said feed piston through the centering ring and into reducing sleeve 8 of headstock 4 of the lathe. In this case, bar 3 with the feed piston may still float in guiding tube 1 whose internal diameter is greater than that of the bar and of the feed piston, respectively.
Also, a telescopic guide-bar device is known from German published application No. 25 33 502, said device having an outer tube which is slidable in a fixed tube of the device, a feed piston rod being slidable in said external tube. Said feed piston rod is provided at its front end with a head which bears against the rear portion of the bar to be machined in order to feed it towards the headstock of the lathe. The diameter of the head is smaller than that of the spindle and of the centering ring which is disposed at the entry of the headstock, such that said head can penetrate into the headstock. The telescopic principle allows to reduce the longitudinal dimension of the device. However, the device according to the abovementioned document is relatively complicated and expensive in conception. The advance of the bar to be machined is controlled pneumatically, without providing a hydrodynamic bearing for supporting the bar, and the rotational speed of the latter is thus limited.